Fuel cell systems for man-portable electrical power supplies, in general, are “battery replacements”, or alternatively, provide a method for recharging a battery while powering an electronic device. Like batteries, fuel cells produce electricity through an electrochemical process, more specifically, a fuel cell produces electricity from fuel and air without combustion. The electrochemical process utilized provides for the combining of hydrogen, the fuel, with oxygen from the air. The process is accomplished utilizing an electrolyte, such as a polymer electrolyte membrane, which conducts ions, such as protons. The polymer electrolyte membrane is sandwiched between two electrodes, namely an anode, the negative electrode used for hydrogen oxidation, and a cathode, the positive electrode used for oxygen reduction. Fuel cells, as known, can perpetually provide electricity as long as fuel and oxygen are supplied. Hydrogen is typically used as the fuel in fuel cells for producing the electricity and it can be processed from methanol, natural gas, petroleum, ammonia, or stored in metal hydrides, carbon nanotubes, or as pure hydrogen.
There are several types of fuel cells, including hydrogen polymer electrolyte membrane cells, Reformed Hydrogen Fuel Cells (RHFCs), Direct Methonal Fuel Cells (DMFCs), and Solid Oxide Fuel Cells (SOFCs).
Hydrogen polymer electrolyte membrane cells are not as suitable for portable applications due to their size, and safety issues caused by carrying around highly volatile hydrogen. Power density of these fuel cells are in the range of 100-200 mW/cm2, making them big for portable applications.
DMFCs have a low power density (typically 30 mW/cm2), high precious metal cost, and are somewhat large for portable operations when more power is needed due to the low power density.
RHFCs utilize hydrogen fuel processed from liquid or gaseous hydrocarbon fuels, such as methanol, using a reactor, called a fuel reformer, for converting the fuel into hydrogen. Methanol is the preferred fuel for use in fuel reformers for portable applications because it is easier to reform into hydrogen gas at a relatively low temperature compared to other hydrocarbon fuels such as ethanol, gasoline, or butane. The reforming or converting of methanol into hydrogen usually takes place by one of three different types of reforming. These three types are steam reforming, partial oxidation reforming, and autothermal reforming. Of these types, steam reforming is the preferred process for methanol reforming because it is the easiest to control and produces a higher concentration of hydrogen output by the reformer, at a lower temperature, thus lending itself to favored use. However, RHFCs also have a low power density (typically 150 mW/cm2) leading to a larger size, an additional reforming unit, and require precious metals, thereby increasing cost. RHFC also requires a low concentration of CO and unreacted hydrocarbons in the reformed gases, so with higher calorific value hydrocarbon fuels such as butane or ethanol, extensive cleanup of the reformed gases will be required. This makes the fuel processor complex and not suitable for portable power applications.
SOFCs comprise a pair of electrodes (anode and cathode) separated by a solid-phase electrolyte. Hydrogen or carbon monoxide pass over the anode reacting with oxygen ions conducted through the electrolyte to produce water and/or carbon dioxide and electrons. The electrons pass from the anode to an external circuit, through a load on the circuit, and back to the cathode where oxygen receives the electrons, thereby converting into oxygen ions which are injected into the electrolyte. SOFC devices may include tubular fuel cells having multiple concentric layers. Inner and outer layers may comprise, respectively for example, an anode and cathode. Fuel may be transmitted through the tube to the anode and oxygen may be supplied to the cathode from outside the tube. The tubes may be stacked so that the sum of the power from each provide a higher powered device. In SOFC, the anode, the electrolyte and the cathode layers can also be arranged in a planar design. However, in such an arrangement there need to be a gas tight seal all around the electrolyte, such that the gases can not leak from the anode side to the cathode side and vice versa. Since these fuel cells also require higher operating temperatures in the range of 700-800° C., accomplishing the seal with good thermal expansion matching of different materials is a major reliability concern. Such planar SOFC devices do not exhibit a thermal robustness. In tubular SOFC, this is less of a concern since the gases flow inside and outside of the tubes and the gas connections and sealing can be accomplished at a relatively cooler ends of the tubes. Such SOFC devices were shown to exhibit good thermal robustness. Tubular design is also volumetrically more efficient design, making it attractive for small fuel cells design in portable power applications.
Fuel reformers have been developed for use in conjunction with fuel cell devices, but they are typically cumbersome and complex systems consisting of several discrete sections connected together with gas plumbing and hardware to produce hydrogen gas, and are thus not suitable for portable power source applications. Recently fuel reformers have been developed, for hydrogen polymer electrolyte membrane cells, RHFCs, and DMFCs devices, utilizing ceramic monolithic structures in which the miniaturization of the reformer can be achieved. Utilizing multilayer laminated ceramic technology, ceramic components and systems are now being developed for use in microfluidic chemical processing and energy management systems. Monolithic structures formed of these laminated ceramic components are inert and stable to chemical reactions and capable of tolerating high temperatures. These structures can also provide for miniaturized components, with a high degree of electrical and electronic circuitry or components embedded or integrated into the ceramic structure for system control and functionality. Additionally, the ceramic materials used to form ceramic components or devices, including microchanneled configurations, are considered to be excellent candidates for catalyst supports and so are extraordinarily compatible for use in microreactor devices for generating hydrogen used in conjunction with miniaturized fuel cells.
Although these fuel reformers using hydrogen polymer electrolyte membrane cells, RHFCs, and DMFCs have shown how to provide power to recharge batteries in electronic devices, there is still a need for higher energy density power sources.